When a three phase motor looses one of the phase supply voltage then it is called Single Phasing. Single Phasing a case when any one phase out of the three phases fails. Since one of the phases is now disconnected, current through other two phases will increase to produce the desired torque. The motor will run but would not be able to drive rated load. The uneven torque results in abnormal noise and vibration in motor.
In order to compensate, the motor starts drawing more amount of current heating the motor even in some cases could burn the motor. Once the motor is started, it continues to run, at least for some more time. What happens in long term depends upon the load.
- If motor is operating at or less than 33%, it will continue to operate without any harm.
- Due to single phasing the current in the remaining two phases increases and it is approximately 2.4 times the normal current value.
- If motor is operating at slightly higher than 33% load, motor will continue to operate, but it will draw current more than its rated value. The motor temperature will start increasing. If thermal overload relay is provided in the motor starter, it will stop the motor before motor burns. But if thermal overload rely is not provided, motor may get burnt.
- If motor is operating at higher load near to the rated load, motor speed will drop gradually to zero. It may take 1 to 30 seconds, depending upon amount of the overload and inertia of the load connected. For high inertia loads, it may take even more time to reduce the speed to zero. In this case thermal overload relay may stop the motor current or fuse may blow. If thermal overload relay / fuses are not provided, motor may get but burnt.